When we want to guarantee that only the authorised addresses can exploit the data transmitted on open networks (cable, satellite, terrestrial broadcast, or Internet), the most appropriate means is to encrypt these data and to ensure that only the authorised addressees have the means for decrypting them.
Despite known encryption algorithms, it is admitted that it is possible for a third person with significant computing power to decrypt these data.
This is why the known systems integrate a frequent key changing mechanism that discourages potential assailants. Each attack then only acts on a small portion of data and only allows access, after decryption, to a few seconds of transmission.
This method is used for the broadcasting of pay television, wherein the key is called the “control word” and only has a duration of a few seconds. These data are referred to as “for immediate consumption” and the corresponding key is sent in parallel.
The appearance of storing means (e.g., digital video recorders) and the possibility of viewing (or exploiting) these data at any given moment has modified the situation somewhat.
In order to further satisfy clients it is now possible to send encrypted data on a distribution network comprising a great number of users. These data are stored in the storage unit of the user unit. These data are accompanied by a file containing the encryption keys, this file also being encrypted according to an algorithm and keys contained in a security module of the user. This security module generally is in the form of a smart card having in its memory its own keys to decrypt the data.
For the following discussion the data defining the product subject to conditional access will be referred to as “product data”. “Product data” means content such as a film, a sports broadcast, a game, or software.
We will consider the fact that a great number of user units contain in their storage unit the same “product data” and a file of keys.
If the user decides to buy this product, the security module will receive the necessary instructions to decrypt the keys file and to supply in useful time the keys to decrypt the “product data”.
An ill-intentioned third person will then attack the keys file that is encrypted by the specific key while the “product data” are encrypted by a great number of keys.
Furthermore, the size of the keys file is small compared with the size of the product data. For information purposes, the size of a film in encrypted form may be about 1 Gigabyte; in decrypted and decompressed form the same can represent a size of 10 Gigabytes.
In this way, when this third person manages to decrypt the keys file this information can circulate easily on the Internet for example, thereby allowing other persons by means of a modified decoder to decrypt the “product data”.
It should be understood that the decoder receives a stream of data whose format depends upon the broadcaster. This means that it is very difficult to isolate the different packages of information to obtain the “product data” in encrypted form at this stage. This is why the attack is made on the storing unit or hard disk, which for economic reasons is of a standard type (IDE for example). This disk is then transferred to a personal computer to receive by other channels the keys file.
In the similar objective of allowing the storing of a product on a hard disk and of viewing it later, a first approach is described in the document FR 2 732 537. The problem that this document seeks to solve is the duration of limited validity of the keys transmitted with the data. This is why the proposed solution is to decrypt the file containing the keys (CW) and to re-encrypt them with a local key to allow thus the use of the data at any time. It should be noted that the data in themselves are stored in the same state as when they entered the decoder.
The embodiment described in the document EP 0 912 052 varies in the sense that the local re-encryption key is stored in a smart card.
These two documents do not allow to solve the problem of the vulnerability of the data when they are stored on a storage device with easy access.